Radiographic inspection of welds for large structural members, such as pipes or large cylinders, is done with a specialized type of equipment consisting of a portable shielded storage or container exposure device which contains a flexible, short wire or "pigtail" containing the radioactive isotope or radioactive source. This radioactive source must be kept shielded at all times when personnel are in the vicinity to avoid harmful exposure to the radiation. The isotope is, therefore, initially mounted to the pigtail at a few isolated laboratories in the United States and stored under heavily shielded conditions. The pigtail then is shipped to the user's location and is again stored until it is ready to be placed into the exposure device, which has a hollow conduit to guide the pigtail to the object to be tested. A flexible wire cable which can be reciprocated from a remote distance, for example, 25-30 feet away, and behind suitable shielding is used to move the pigtail through the exposure device thence through the hollow conduit, and into the vicinity of the object to be radiographed.
The shipping container containing a pigtail, with its new radioactive isotope, is coupled to the exposure device; the cable is then pushed into the exposure device and screwed to the fresh isotope-carrying pigtail; then the hollow conduit is substituted for the shipping container.
When testing pipe welds, etc., the free end of the conduit is usually brought within the center of the cylinder or pipe to be inspected. Then, by pushing the cable through the exposure device the pigtail having the radioactive isotope are pushed through the conduit to the end of the conduit. Radiographic film is on the outside of the container in the area to be X-rayed. If desired, an additional shielding block can be placed above the isotope in the container to reduce scattering of the radioactivity. When the desired amount of radiation has been emitted to expose the film, the pigtail is withdrawn by the cable back into the exposure device until is needed again.
During movement of the pigtail in the conduit, the pigtail becomes weakened and occasionally breaks. If the pigtail breaks outside the exposure device there is considerable danger of overexposure of personnel to radiation. Since the testing is frequently done in an operating plant of some kind, this risk of overexposure of personnel may cause the plant or a substantial part of the plant to be shut down until the broken pigtail can be placed in a shielded container.
Heretofore, inspection of the pigtail for wear or potential breakage has been done haphazardly, with perhaps the best known equipment being the use of a mirror, auxiliary lighting and a remotely positioned telescope located in a shielding viewing area. The resolution, necessity of lighting, and the handling of the mirror for this inspection have never been satisfactory. The equipment is cumbersome and awkward to position and use.